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From: Dave Hansen <dave.hansen@intel.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H. Peter Anvin" <hpa@zytor.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Andy Lutomirski <luto@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/mm: Enable 5-level paging support by default
Date: Fri, 13 Sep 2019 11:20:26 -0700
Message-ID: <8435951c-d88a-a5c6-5328-90c9f2521664@intel.com> (raw)
In-Reply-To: <20190913095452.40592-1-kirill.shutemov@linux.intel.com>

On 9/13/19 2:54 AM, Kirill A. Shutemov wrote:
> The next major release of distributions expected to have
> CONFIG_X86_5LEVEL=y.

It's probably worth noting that this exposes to two kinds of possible
performance issues:

First is the overhead of having the 5-level code on 4-level hardware.
We haven't seen any regressions there in quite a while.  Kirill talked
about this in the changelog.

Second is the overhead of having 5-level paging active on 5-level
hardware versus using 4-level paging on hardware *capable* of 5-level.
That is, of course, much harder to measure since 5-level hardware is not
publicly available.  But, we've tested this quite a bit and we're pretty
confident that it will not cause regressions, especially on systems
where apps don't opt in to the larger address space.

I do think endeavoring to have mainline's defaults match the most common
distro configs is a good idea, and now is as good of a time as any.

Acked-by: Dave Hansen <dave.hansen@linux.intel.com>


  reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-13  9:54 Kirill A. Shutemov
2019-09-13 18:20 ` Dave Hansen [this message]
2019-09-16 14:53   ` Ingo Molnar
2019-09-16 14:54 ` [tip: x86/mm] " tip-bot2 for Kirill A. Shutemov

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